Embodiments of the invention relate generally to magnetic resonance (MR) imaging and, more particularly, to the parallel transmission of radio-frequency (RF) pulses during a spin echo sequence.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. As appreciated by those skilled in the art, one or more radio-frequency (RF) pulses are generally employed to create the excitation field, B1, which is applied to the substance or tissue, thus manipulating an ensemble of spins thereof.
After application of the B1 excitation field, a signal is emitted by the ensemble of spins is acquired and processed to form an image. Depending on the technique employed, the ensemble of spins may be subjected intervening acts prior to acquisition of the image signal. For example, with gradient echo imaging, one or more gradient reversals are employed to produce the image signal. Alternatively, with regard to spin echo imaging, one hundred and eighty degree RF pulses are generally employed to create the image signal. Regardless of the imaging technique employed, the resulting set of received NMR signals is digitized and processed to reconstruct the image.
Due, in part, to the complexity of MR imaging, image artifacts may be present in the reconstructed images. Image artifacts can have a variety of causes. For example, inhomogeneity in the B1 excitation field produced by an RF pulse can cause image artifacts such as the shading artifact.
It would therefore be desirable to have a system and method capable of minimizing inhomogeneity in the B1 excitation field.